1. Field of the Invention
The present invention relates to a novel technique for the planarization of filled trenches. This technique involves isotropically etching SiO.sub.2 -filled trenches and resist and chemical mechanical polishing (CMP) without polishing to the "active" regions on the wafer surface.
2. Description of the Related Art
Etching trenches into silicon wafers in order to electrically isolate the various active regions of an electronic circuit is a well known technique. Because it is desirable to facilitate future processing, to maintain a planar upper surface on the wafer, chemical vapor deposition (CVD) techniques are used to fill trenches with an electrically insulating material such as SiO.sub.2. However, CVD deposits oxide in both trenches and on top of pillars in the active regions (e.g., as shown in the exemplary pattern in FIG. 1a) Therefore some kind of planarization procedure must be used to achieve nominally flat top surfaces. Chemical mechanical polishing (CMP) of the as-deposited material is perhaps the simplest approach, but this technique suffers from local polish rate variations as well as dishing and problems associated with oxide residues. These variations generally may be said to be caused by the finite stiffness of the pads conventionally used for CMP, variations of the local pattern density and because different materials etch at different rates during CMP.
For example, the local etch rate strongly depends on the pattern of nearby trenches 10 and, specifically, the density of these patterns. One problem caused by nonuniformities in local etch rates is "dishing" in wide open trench areas 12, where trench areas and nontrench areas etch at different rates. Because of pad flexibility, some filler residue may be left on top of active areas between trenches, detracting from flatness.
Another problem is the rapid erosion of small isolated non-trench features ("pillars" 14) in wide open trench areas. The pad flexibility over such isolated pillars leads to fairly localized polishing action so that there is accelerated erosion, especially at the upper corners of the pillars.
Although many of the disadvantages of CMP are related to pad flexibility, increasing the stiffness of the pads is not generally a useful solution. Some pad flexibility is needed in order for the pad to conform to gross wafer bowing.
For these reasons, CMP of CVD oxide deposited on pillars and in trenches of varying widths and lengths is not generally practiced or recommended.
More advanced planarization processes involve an additional lithography step that essentially protects material in the trench regions while exposing material in the non-trench (active) regions. An etch is then performed that removes the bulk of the filler oxide on top of the pillars (the "pillar oxide"). Typically, this etch is an anisotropic plasma etch that leaves "ears" 16 and 18 of filler material, exemplified in FIG. 1b. While a significant amount of filler material has been removed from the active regions in such an etch step, a significant fraction in the form of ears remains. When this step is followed by CMP, there is still potential for pattern-dependent polishing as described above.
U.S. Pat. No. 5,498,565, issued to Gocho and assigned to Sony Corp., offers some improvement by using an isotropic (as opposed to an anisotropic) oxide etch to remove an even greater fraction of pillar oxide. Gocho describes a method of filling trenches by simultaneously depositing and etching (i.e., a ECR-CVD process) a CVD oxide. The deposited layer is then masked with resist, isotropically etched and then polished using a CMP technique. Gocho teaches a technique that is superior to the prior art with respect to reduced pattern sensitivity of the final polishing step through the use of an isotropic oxide etch and more efficient removal of pillar oxide.
However, Gocho fails to teach how to overcome problems of isolated resist line lift-off. An isotropic etch, as Gocho proposes, will undercut the resist mask. If this occurs on both sides of a narrow resist line, then it is possible to completely undercut, and thereby liberate, the resist line. The line is then free from the surface and can lift off, fall over or land on another part of the substrate, interfering with isotropic etching where it lands.
It would therefore be desirable to have a simple and effective trench planarization technique with lower pattern sensitivity capable of use with small geometries.